Metal hot-dip coating technology
Hot-dip metal coating is a surface treatment method that involves immersing the workpiece in molten coating metal to form a firmly bonded coating. The basic principle is to use high temperatures to induce diffusion and chemical reaction between the coating metal and the base metal, forming an alloy layer and achieving a metallurgical bond between the coating and the base metal. Hot-dip coating offers advantages such as uniform coating thickness, strong adhesion, excellent corrosion resistance, and high production efficiency. It is widely used in the corrosion protection of steel products such as galvanized steel sheets, galvanized steel pipes, and highway guardrails. Hot-dip galvanizing is the most commonly used hot-dip coating process, accounting for over 90% of total hot-dip coating output.
The main processes of metal hot-dip plating technology include pretreatment, hot-dip plating, and post-treatment. The purpose of pretreatment is to remove impurities such as grease, rust, and scale from the workpiece surface to ensure good bonding between the coating and the substrate. Pretreatment generally includes degreasing, pickling, and flux plating. Degreasing can be done chemically or at high temperatures; pickling uses hydrochloric acid or sulfuric acid solutions to remove rust and scale; flux plating involves immersing the workpiece in a flux (such as zinc chloride or ammonium chloride solution) to form a protective film. This prevents re-oxidation before the workpiece enters the molten metal and improves the wettability of the coating metal on the workpiece surface.
Hot-dip plating is the core process, where the pretreated workpiece is immersed in molten coating metal (e.g., the melting point of zinc is approximately 419°C, and the melting point of aluminum is approximately 660°C) for a specified period of time before removal. During the hot-dip plating process, the workpiece surface comes into contact with the molten metal, causing a series of physical and chemical changes: first, the coating layer on the workpiece surface decomposes, revealing a clean metal surface; then, the molten metal wets and spreads across the workpiece surface; then, the base metal and coating metal diffuse into each other, forming an alloy layer; finally, a layer of pure coating metal covers the alloy layer. Parameters such as the coating temperature, coating time, and coating metal composition affect the coating’s thickness, structure, and performance, requiring precise control based on the workpiece material and intended use.
In metal hot-dip plating technology, common coating metals include zinc, aluminum, zinc-aluminum alloys, and aluminum-magnesium alloys. Each coating has its own unique properties and application scenarios. Hot-dip galvanizing offers excellent corrosion resistance and low cost, making it suitable for steel products exposed to atmospheric and freshwater environments. Hot-dip aluminum coatings offer excellent high-temperature and corrosion resistance, making them suitable for high-temperature environments (such as boilers and heat exchangers) and highly corrosive environments (such as chemical equipment). Zinc-aluminum alloy coatings combine the sacrificial anodic protection of zinc with the passivation protection of aluminum, offering superior corrosion resistance compared to pure zinc coatings. They are widely used in automotive, home appliance, and other fields.
With the development of industrial technology, metal hot-dip plating technology continues to advance. The development of new coating alloys (such as zinc-aluminum-magnesium alloys) has further improved the corrosion resistance and processability of the coating. The automation level of continuous hot-dip plating lines continues to increase, achieving full automation of the entire process from workpiece loading to coating inspection, improving production efficiency and the stability of coating quality. The application of environmentally friendly pretreatment processes (such as chromium-free passivation and environmentally friendly plating fluxes) has reduced pollutant emissions. In the future, metal hot-dip plating technology will continue to develop towards high performance, high efficiency, and environmental friendliness, providing better solutions for the long-term protection of steel products.
